1. Field of the Invention
The present disclosure relates to directional couplers, and, in particular, a directional coupler including a main line and a sub line.
2. Description of the Related Art
For example, as one of directional couplers in related art, a directional coupler disclosed in Japanese Unexamined Patent Application Publication No. 9-153708 (hereinafter, patent document 1) is known. This directional coupler includes a plurality of dielectric layers, a first coupling line, a second coupling line, a first ground electrode, and a second ground electrode.
The plurality of dielectric layers has a substantially rectangular shape, and the layers included therein are stacked on top of each other in an up-and-down direction. The first coupling line and the second coupling line are provided on the dielectric layers that are different from each other, and electromagnetically couple to each other. The first coupling line is located above the second coupling line. The first ground electrode is provided on a dielectric layer located above the first coupling line. The second ground electrode is provided on a dielectric layer located below the second coupling line. The directional coupler configured as described above achieves downsizing and low insertion loss, and has a sufficient isolation characteristic.
However, in the directional coupler described in the patent document 1, it is possible to have a difference between the characteristic impedance of the first coupling line and the characteristic impedance of the second coupling line. Specifically, in the directional coupler, there may be manufacturing variation in thickness in the plurality of dielectric layers. Therefore, there may be variation between the distance from the first coupling line to the first ground electrode and the distance from the second coupling line to the second ground electrode. This may cause a deviation from a preset capacitance to be formed between the first coupling line and the first ground electrode and a deviation from a preset capacitance to be formed between the second coupling line and the second ground electrode, and there may be variation between the amount of the former deviation and the amount of the latter deviation. As a result, a difference is likely to occur between the characteristic impedance of the first coupling line and the characteristic impedance of the second coupling line.